1. Field of the Invention
The invention relates to the provision of an insulating coating on an object of steel, especially silicon steel.
2. The Prior Art
When manufacturing sheet or strip material of silicon steel, so-called electrical sheet or strip, the material is subjected after rolling to a heat treatment at a temperature of from about 850.degree. to 1350.degree.C in order to produce grain growth of the crystals which is necessary if the sheet or strip material is to obtain the necessary magnetic properties.
Before the heat treatment, the sheet or strip material is coated with chemicals which during the heat treatment are to form an electrically insulating protective layer on the material. One such known protective layer may consist of a reaction product of silicon dioxide formed on the surface of the sheet or strip material and an oxide or hydroxide of an alkaline earth metal applied thereon, usually magnesium, or of essentially unreacted alkaline earth oxide. The application of the protective layer on the surface of the material is performed by suspending the alkaline earth metal oxide or hydroxide in water, then coating it on to the sheet or strip material in an even layer, after which the sheet or strip material is subjected to the heat treatment previously mentioned, at a temperature of from 850.degree.C to 1,350.degree.C in hydrogen atmosphere, and if a well-developed glass film is to be formed on the sheet or strip material the temperature should amount to about 1,000.degree.C to 1,350.degree.C. The hydroxide which is included in the suspension from the start or which is formed from the oxide by reaction with water, liberates water during the heating of the sheet or strip material, which water, at temperatures below the last mentioned range, is able to oxidize silicon in the steel to silicon dioxide without the iron being oxidized at the same time. The oxide which is formed from the hydroxide during the liberation of water, or which was possibly added from the start and has avoided hydration, reacts with the silicon dioxide at temperatures in the range from about 1,000.degree. to 1,350.degree.C, thus forming a well-developed glass film on the surface of the sheet or strip material, as mentioned earlier. The glass film can also be produced by the use of a carbonate of an alkaline earth metal. The carbon dioxide which the carbonate liberates upon being heated can oxidize silicon to silicon dioxide without the iron being oxidized. When the silicon dioxide has been produced, the formation of the glass continues in the manner described above. Any excess of oxide which has not reacted during the glass formation acts as spacer material between adjacent layers of the sheet or strip material whether these occur as turns in a roll or as laminae in a stack, thus preventing the layers from sticking or sintering together.
The process described above involving heat treatment at a temperature in the range from about 1,000.degree. to 1,350.degree.C, is normal in the manufacture of silicon steel with grain orientation where the formation of a glass film is particularly important.
Silicon steel without grain orientation normally contains a few tenths percent by weight aluminium, which means that no real glass film is formed during the heat treatment, even if this is carried out in the temperature range of from 1,000.degree.C to 1,350.degree.C. The alkaline earth metal oxide instead remains as a layer serving principally as a spacer material. This is quite sufficient as a protective layer for silicon steel without grain orientation. In this case, it is also possible to replace the oxide or hydroxide of the alkaline earth metal normally used, by aluminium oxide or hydroxide.
A protective coating of silicate of the kind described above has an electrical insulating resistance which is unsatisfactory for many purposes, and therefore the protective coating is often reinforced by treating it with phosphoric acid or metal phosphates, for example according to the methods which are described in the Swedish Pat. specification 129,585. Such a treatment with phosphoric acid or phosphates can also be made directly on the pure sheet material.
When applying phosphate on said protective coating of silicate, the protective coating may become porous. This is due to the phosphate penetrating into the silicate layer which loosens it. The penetration of the phosphate also causes a deterioration of the otherwise good adhesion of the coating to the sheet and also a deterioration of the tenacity of the coating and therefore of its resistance to, for example, bending. The loosening of the coating also means that it can easily be burst away from the sheet in the subsequent annealing at a temperature of about 800.degree.C which is carried out in order to remove inner tensions in the sheet, for there is a risk of gases penetrating into the sheet material because of the porosity of the coating and causing the unfavourable effect. In the subsequent annealing in hydrogen atmosphere of phosphate coatings, the phosphate layer may be reduced, thus strongly imparing the insulating resistance.
The protective layer which is obtained by treating a pure sheet material with phosphate becomes porous and shows bad adhesion to the sheet material. The porosity is a disadvantage with regard to the corrosion resistance as well as with regard to insulating properties.